To facilitate the handling of sheets such as paper sheets which are ejected from an imaging device such as photocopiers and printers, it is desirable to use an offset stacker that stacks the sheets on a stack table as individual sets that are transversely offset from one set to another. Japanese patent laid open publications JP2003-312931A (patent document 1) and JP2003-341908A (patent document 2) disclose such offset stackers.
In such an offset stacker, typically, a transversely moveable jogger is used for jogging of pushing each sheet to a transversely offset position with respect to the transport direction of the sheet. See patent document 1, for instance.
However, according the prior art, as the jogger pushes each sheet transversely, the offset distance could vary from one sheet to another owing to the unevenness in the frictional property between the sheets. Therefore, the side edges of the sheets in each set may not be lined up evenly, and this may cause some inconvenience in the subsequent handling of each set of sheets.